1. Field
The embodiments discussed herein relate to the field of cellular communication networks, and more precisely to a method to balance traffic load between nearby LTE/WiMAX cells grouped into inner and border constellations.
2. Description of the Related Art
Long Term Evolution (LTE) and WiMAX systems being defined by 3GPP and IEEE/WiMAX Forum, respectively, are characterized by a simplified Radio Access Network (RAN) structure in which the functions of the centralized controller, e.g. the RNCs in the UTRAN, are partially shifted to the base stations in order to reduce latencies and improve the quality of service (QoS) provided to the end user. The results are enhanced Nodes B (eNB) in the LTE architecture and enhanced Base Stations BS in WiMAX. Both architectures allow base station to base station communication for coordination purposes. The LTE provides this via specific interfaces between eNBs (X2), while current WiMAX standards rely on the availability of a mediation function provided by another Access Serving Network (ASN) Network Element (ASN-GW), since a BS to BS interface (R8 reference point) is not clearly defined yet.
Existing WiMAX specifications define a hierarchical Radio Resource Management (RRM) architecture in which a Radio Resource Controller (RRC) checks the radio resource utilization of Radio Resource Agents (RRA). RRC entities may be mapped to base stations or to centralized controllers (ASN-GWs) and may coordinate each other via the ASN-GW. The present FIG. 1 is reported or obtained from the following draft: WiMAX Forum stage 2 specification: “FIG. 7-92—RRA and RRC Collocated in BS”.
The present FIG. 2 is reported or obtained from the following WiMAX Forum stage 3 specification: “FIG. 5 26—Inter-ASN RRM Communication is RRC to RRC Communication. Note: A similar figure occurs for intra-ASN R4 communication; two ASN GWs within a single ASN communicate via R4; from RRM view, this is RRC-RRC communication like on inter-ASN R4”.
The present FIG. 3 is a Table reported or obtained from the following draft: WiMAX Forum stage 3 specification: “Table 5 43—RRM Procedures, Messages, Mapping to Reference Points”.
As far as the WiMAX profiles concept is concerned, making reference to the preceding figures, WiMAX Forum stage 2 defines three Access Serving Network (ASN) profiles, which correspond to three distinct mapping of functionality to BS and ASN-GW network elements. In this context we are just interested in the Radio Resource Management (RRM) functionality, although the profile concept considers other functional mappings as well. In Profile A, the ASN-GW is in charge of centralized RRM. In Profile C, no central RRM is defined and each BS has a local RRM entity. Profile B does not define a specific functional to physical mapping, allowing the equipment manufacturer to decide. The profile concept is defined only for WiMAX but not for LTE.
The present FIG. 4 is reported or obtained from the following draft: WiMAX Forum stage 3 specification: “FIG. 5-27—Per-BS Spare Capacity Reporting Procedure”.
FIGS. 5 and 6 represents current network architectures for WiMAX (Profile C) and LTE, respectively. With reference to FIG. 5, we see two WiMAX base stations BS1 and BS2 of the type reported in FIG. 1 interfaced to a first network element ASN GW1 through R6, and a third base station BS3 interfaced to a second network element ASN GW2 also through R6; the two ASN GW1 and ASN GW2 are interfaced through R4. All the depicted elements BS1, BS2, BS3, ASN GW1, and ASN GW2 are interconnected by an IP backbone encompassing the R6 and R4 connections. With reference to FIG. 6, we see two LTE base stations eNB1 and eNB2 directly connected to each other through an X2 interface and to a centralized controller Access GW through respective S1 interfaces. Also in LTE all depicted elements eNB1, eNB2, and Access GW are interconnected by an IP backbone encompassing the X2 and both S1 connections.
The international patent application PCT/EP2006/06033 filed by the same Applicant on 23 Jun. 2006 claims (for the common subject matter) the priority date of the european patent application EP 05425456.0 filed on 24 Jun. 2005, which in its turn constitutes prior art under the Article 54(3) EPC. PCT/EP2006/06033 claims a method for sharing signaling load between radio access network nodes (RNC/eNB), belonging to a mobile radio communication system (UMTS), via a transport network (Cluster Backbone) interconnecting said nodes, where each node processes the Radio Resource Control signaling to serve voice calls and/or packet data transmissions and measures processing load, periodically, in order to detect an incoming congestion state of the internal processing means, characterized in that includes the following steps:                a) spreading around processing capacity status information from each node to other nodes of a cluster of nodes constituting an independent signaling processing pool;        b) issuing a processing capacity request from a congested or nearby congested node to a target node of said cluster having residual processing capacity;        c) rerouting the incoming signaling from the requesting node to the target node which accepts said request as serving node;        d) processing the rerouted signaling by the serving node on behalf of the requesting node;        e) rerouting the outcome of the processed signaling from the serving node to the requesting node.        
FIG. 7 shows the network infrastructure to carry out the method for sharing signaling load disclosed in the aforementioned PCT/EP2006/06033; it resembles the preceding FIG. 6 with the only exceptions that the IP backbone includes the only X2 interface between eNB1 and eNB2 for transmitting Processing Status signaling. PCT/EP2006/06033 differs from EP 05425456.0 by the only fact that the involved radio access network nodes also include peer eNBs of the future LTE system.
The following open issues exist:                The WiMAX standard does not define means for performing complex RRM tasks like traffic load balancing. The existing primitives (Spare capacity request/report) may not be sufficient:        they allow RRC to be informed of the load status of controlled and neighbor cells but,        they do not provide a way for preventing the flooding of unloaded cells with contemporaneous requests made by several RRCs. When several RRCs detect an unloaded part of the system, they may contemporaneously decide to offload their excess traffic to it, thereby causing an overload condition.        The WiMAX standard presently does not describe in detail the communication between RRCs located in BSs connected to the same ASN-GW.        
We are not aware of solutions described in current LTE standards (in progress); sections 15.2.3 “De centralized RRM” and 15.2.4 “Load balancing control” of TS 25.xxx “E UTRAN overall description stage 2” are currently empty and section 6.12.3 “RRM architecture in LTE” of TR R3.018 “Evolved UTRA and UTRAN radio access architecture and interfaces” provides a generic overview only.
Since in the LTE architecture, no centralized radio resource control functionality is available, any optimization task, like e.g. advanced Radio Resource Management (RRM) techniques, has to rely on a RRM function distributed among the available eNBs. WiMAX Profile C, in which no centralized RRM controller exists, experiences a similar situation.
On the other hand, the teaching of PCT/EP2006/06033, although referred to peer eNBs, is exclusively directed to balance Layer 3 signaling processing but not radio resources. An overload of signaling processing is not mandatorily tied to traffic overload and, in any case, the action taken by a relaxed processor is to take over signaling from a busy one with transparency, but not to promote traffic load balance.